Microwave circuit

ABSTRACT

A microwave circuit comprises a waveguide which prevents the passage of the idler frequency. A coaxial line extends from the waveguide substantially perpendicular to the waveguide. A varactor is mounted in the waveguide at the intersection of the waveguide and the coaxial line. Short-circuit means provides an equivalent short-circuit surface in the waveguide at approximately a quarter wavelength from the diode. An idler circuit comprises the inductance exhibited by the waveguide. Idler circuit adjusting means is provided in the waveguide at more than a quarter wavelength of the idler frequency for adjusting the idler frequency. The idler circuit is a resonant circuit which resonates at a frequency higher than the selfresonant frequency of the diode.

451 Apr. 16, 1974 MICROWAVE CIRCUIT Tatsuo Kudo, Kawasaki; KoichiKurachi, Yokohama, both of Japan Inventors:

Fujitsu Limited, Kawasaki, Japan Filed: Mar. 9, 1973 Appl. No.: 339,882

Related US. Application Data Continuation-impart of Ser. No. 162,340,July 14, 1971, abandoned.

Assignee:

Foreign Application Priority Data Primary Examiner1-1erman Karl SaalbachAssistant Examiner-Darwin R. Hostetter Attorney, Agent, orFirm--l-Ierbert L. Lerner [57] ABSTRACT A microwave circuit comprises awaveguide which prevents the passage of the idler frequency. A coaxialline extends from the waveguide substantially perpendicular to thewaveguide. A varactor is mounted in the waveguide at the intersection ofthe waveguide and the coaxial line. Short-circuit means provides anequivalent short-circuit surface in the waveguide at approximately aquarter wavelength from the diode. An idler circuit comprises theinductance exhibited by the waveguide. Idler circuit adjusting means isprovided in the waveguide at more than a quarter wavelength of the idlerfrequency for adjusting the idler frequency. The idler circuit is aresonant circuit which resonates at a frequency higher than theself-resonant frequency of the diode.

5 Claims, 21 Drawing Figures CHOKE" PM 752 4 IOLER (7260/7 40.105774 6Elf/WENT r5 TRANS/02,4452 7 coax/42 Cali meme 6' MICROWAVE CIRCUIT Thepresent application is a continuation-in-part of pending applicationSer. No. 162,340, filed July 14, 1971 now abandoned, for microwavecircuit, assigned to the assignee hereof.

The invention relates to a microwave circuit. More particularly, theinvention relates to a microwave circuit having an idler resonantcircuit, which microwave circuit may be utilized in a parametricamplifier, a multiplier, a frequency converter, or the like.

A parametric amplifier is a microwave amplifier having as its basicelement an electron tube or solid state device whose reactance can bevaried periodically by an AC voltage at a pumping frequency. Operationis at room temperature. The diode amplifier, ferromagnetic amplifier,and up-converter are examples. The parametric amplifier is also calledmavar, paramp and reactance amplifier.

In a conventional parametric amplifier, the selfresonance of a variablecapacity diode such as, for example, a varactor diode utilized as theamplifying element, is generally utilized as the idler circuit in orderto widen the bandwidth of the parametric amplifier. A varactor is asemiconductor device characterized by a voltage-sensitive capacitancewhich resides in the space-charge region at the surface of asemiconductor bounded by an insulating layer. A varactor may be utilizedin automatic frequency control and electronic tuning circuits, and forparametric amplification. A varactor is also called varicap andvoltage-variable capacitor.

The self-resonance of a varactor diode may be di' vided into a firstself-resonance having a frequency frl which is available when theimpedance viewed-from the varactor diode is zero, that is, when thevaractor diode is short-circuited, and a second self-resonance having afrequency fr2 available when the outside impedance viewed from thevaractor diode is infinite, that is, when the varactor diode isopen-circuited. Generally, the frequency fr2 of the secondself-resonance is larger than the frequency fr] of the firstself-resonance. In the case of a varactor diode utilized in a parametricamplifier for microwaves, the frequency fr2 is approximately 1.4 to 2times frl. I

When the first self-resonance of the varactor diode is utilized as theidler circuit, the conventional parametric amplifier has a narrowbandwidth, an uneven and complicated amplification characteristic curve,and occasional oscillation, and the capability of the varactor dioderelative to the noise characteristic cannot be fully utilized. When thesecond self-resonance of the varactor diode is utilized as the idlercircuit, the conven tional parametric amplifier has a spurious response,produces an uneven and complicated amplification characteristic curveand sometimes oscillates.

An object of the invention is to provide a microwave circuit whicheliminates the disadvantages of the conventional parametric amplifiers.

Another object of our invention is to provide a microwave circuit whichfacilitates the adjustment of the idler circuit of a parametricamplifier.

Another object of the invention is to provide a microwave circuit whichprovides a wide bandwidth of the parametric amplifier, eliminates thespurious response of the parametric amplifier, produces an even anduncomplicated amplification characteristic curve, eliminatesoscillations, and permits the full utilization of the capability of thevaractor diode in relation to the noise characteristic.

Still another object of our invention is to provide a microwave circuithaving an idler resonant circuit which may be readily adjusted, withless deterioration of the frequency band characteristic and with anexcellent noise characteristic.

Still a further object of the invention is to provide a microwavecircuit which permits the operation of the parametric amplifier at thepoint at which the noise characteristic of the varactor diode is themost suitable by utilizing an idler frequency higher than theselfresonant frequency of the varactor diode, confining the energy ofthe idler frequency in an area very close to the varactor diode andwidening the amplification bandwidth, and adjusting the idler circuitwith facility and rapidity.

Still another object of our invention is to provide a microwave circuitwhich enables a parametric amplifier operated at room temperature toproduce a sufficiently wide bandwidth and reduces the noise temperatureat the ends of the band to a minimum.

A further object of the invention is to provide a mi crowave circuitwhich permits a parametric amplifier to function with efficiency,effectiveness and reliability.

In accordance with the invention, a microwave circuit comprises awaveguide which prevents the passage of the idler frequency/Thewaveguide exhibits an inductance. A coaxial line extends from thewaveguide substantially perpendicular to the waveguide. A varac tor ismounted in the waveguide at the intersection of the waveguide and thecoaxial line. Short-circuit means provides an equivalent short-circuitsurface in the waveguide at approximately a quarter wavelength from thediode. An idler circuit comprises the inductance exhibited by thewaveguide. Idler circuit adjusting means is provided in the waveguide atmore than a quarter wavelength of the idler frequency for adjusting theidler frequency.

The waveguide exhibits an inductance in the area of the varactor diode.The coaxial line exhibits a reactance in the area of the diode. Thediode has a selfresonant frequency. The idler circuit comprises theinductance exhibited by the waveguide in the area of the diode and thereactance exhibited by the coaxial line in the area of the diode. Theidler circuit is a resonant circuit which resonates at a frequencyhigher than the self-resonant frequency of the diode.

The idler circuit adjusting means may comprise a screw inserted into thewaveguide, or a pair of spaced screws inserted into the waveguide.

The idler circuit adjusting means may comprise a first screw insertedinto the waveguide at a first specific distance from the diode andextending into the waveguide to a first variable depth and a secondscrew inserted into the waveguide at a second specific distance from thediode and extending into the waveguide to a second variable depth.

A radial choke is provided between the waveguide and the coaxial linefor choking the idler electric power. Fine adjusting means for the idlercircuit comprises a disc having a concave groove formed therein adjacentthe coaxial line between the radial choke and the varactor diode. Thefine adjusting is determined by the depth of the groove. The disc variesthe electrical length between the choke and the diode.

A Iowpass filter between the waveguide and the coaxial line has a lowpass filter element for preventing the leakage of electric power to thecoaxial line. Fine adjusting means for the idler circuit comprises aconcave groove formed in the filter element. The fine adjusting isdetermined by the depth of the groove.

Support means supports the varactor diode in the waveguide. A low passfilter is provided adjacent the diode between the waveguide and thecoaxial line. Fine adjusting means for the idler circuit comprises aconcave groove formed in the support means in the waveguide adjacent thediode. The fine adjusting is determined by the depth of the groove.

In order that the invention may be readily carried into effect, it willnow be described with reference to the accompanying drawings, wherein:

FIG. la is a sectional view of an embodiment of a conventionalparametric amplifier;

FIG. lb is the equivalent circuit of the idler circuit of the embodimentof the parametric amplifier of FIG. 1a;

FIG. 2a is a sectional view of another embodiment of a conventionalparametric amplifier;

FIG. 2b is the equivalent circuit of the idler circuit of the embodimentof the parametric amplifier of FIG. 2a;

FIG. 3a is a sectional view of an embodiment of a parametric amplifierutilizing the microwave circuit of the invention;

FIG. 3b is the equivalent circuit of the idler circuit of the embodimentof the parametric amplifier of FIG. 3a;

FIG. 4a is a sectional view of another embodiment of a parametricamplifier utilizing another embodiment of the microwave circuit of theinvention;

FIG. 4b is the equivalent circuit of the idler circuit of the embodimentof the parametric amplifier of FIG. 40;

FIG. 5 is a graphical presentation of the frequency characteristics ofthe conventional parametric amplifiers and the parametric amplifiers ofthe invention;

FIGS. 6a, 6b, 6c and 6d are schematic diagrams of the waveguide of themicrowave circuit of the invention for explaining the operation of theinvention;

FIG. 7 is the equivalent circuit of the idler circuit of anotherembodiment of the parametric amplifier of the invention;

FIG. 8 is a graphical presentation of the relation between the idlerfrequencies and the bandwidths and noise temperatures of the idlercircuit;

FIG. 9 is a graphical presentation of the relation between the signalfrequencies and the gain and noise temperatures of the parametricamplifiers;

FIGS. 10a and 10b are sectional views of an embodiment of the parametricamplifier of the invention;

FIG. 11a is a sectional view of part of another embodi-ment of theparametric amplifier of the invention;

FIG. 11b is a sectional view of part of still another embodiment of theparametric amplifier of the invention; and I FIG. 12 is a graphicalpresentation for explaining the characteristics of the embodiments ofFIGS. 10a, 10b and 11a, 11b.

In the figures, the same components areidentified by the same referencenumerals.

The microwave circuit of the present invention is described withreference to parametric amplifiers utilizing microwave circuits.

FIG. la illustrates the parametric amplifier constituted when the firstself-resonance of the varactor diode is utilized as the idler circuit.The parametric amplifier of FIG. la comprises a varactor diode 1. Anelectric power supply pumping waveguide 2 is reduced in its verticaldimension or height in the electric field direction from the normalvertical dimension of a waveguide 3 by a tapered portion. The waveguide2 is also reduced in its horizontal dimension or width in the directionperpendicular to the electric field direction. The reduction indimensions permits the transmission of the pumped electric power, butprevents the transmission of the idler frequency through the waveguide2.

A choke filter 4 is tuned to the idler frequency and is spaced from awide wall 5 of the waveguide 2 by a distance equal to a quarterwavelength of the idler frequency. The impedance is made sufficientlysmall in value by the idler frequency at the position of the wide wall 5of the waveguide 2, when the filter 4 is viewed from the varactordiode 1. A coaxial conductor having an inner conductor 6 is providedperpendicularly to the waveguide 2 and operates to provide the tuning ofthe reactance of the varactor diode with the signal frequency.

A transformer 7 varies the coupling coefficient of the signal circuit.The coaxial conductor has a signal input and output end 8. The signalinput and output end 8 is connected to a circulator (not shown in theFIG.) for separating the input and output. A variable shortcircuit end 9is provided for the waveguide 2, matching said waveguide and thevaractor diode I. The end 9 is an arbitrary distance L2 from thevaractor diode 1.

FIG. lb shows the equivalent circuit of the idler resonant circuit ofthe parametric amplifier of FIG. 1a. The equivalent circuit of FIG. lbcomprises an inductance l1 and a series circuit arrangement 12 of acapacitor and an inductance connected in series with the varactordiode 1. The inductance 11 indicates that the waveguide 2 of FIG. 1a,which prevents the passage or propagation of the idler frequency, may beregarded as an inductance. The series circuit arrangement 12 of FIG. lbindicates that the filter 4 of FIG. la, viewed from the wide wall 5 ofthe waveguide 2, may be regarded as a series resonant circuit.

When the second self-resonance of the varactor diode is utilized as theidler circuit, the parametric amplifier is constituted as shown in FIG.2a. The embodiment of FIG. 2a is different from that of FIG. 1a in thespacing of the filter 4. In the embodiment of FIG. 2a, the filter 4 isspaced from the wide wall 5 of the waveguide 2 by lessthan one tenthwavelength of the idler frequency. The choke filter 4 is thus closer tothe wide wall 5 of the waveguide 2 in FIG.- 2a.

The equivalent circuit of FIG. 2b includes the inductance l1 and aparallel resonant circuit 13 having a capacitor connected in parallelwith an inductor. The parallel resonant circuit 13 is connected inseries with the varactor diode l. The equivalent circuit of FIG. 2bindicates that the filter 4, viewed from the wide wall 5 of thewaveguide 2, may be regarded as the parallel resonant circuit 13, andthat the parallel resonant circuit 13 is coupled to the inductance 11,as indicated by a double-headed arrow 14.

The equivalent inductance ll of the waveguide 2, through which the idlerfrequency cannot be passed in the conventional parametric amplifier ofFIGS. 1a and 2a, is substantially unnecessary and must therefore be madeas small as possible. It has, however, been difficult to make theinductance ll sufficiently small without a disadvantageous influence onthe signal circuit or pump circuit. The inductance ll narrows thebandwidth of the parametric amplifier of FIG. 1a and causes a spuriousresponse of the parametric amplifier of FIG. 2a. Furthermore, theinductance 11 produces an uneven and complicated amplificationcharacteristic curve and sometimes causes oscillations.

The parametric amplifier of FIG. 1a has an additional disadvantage thatthe self-resonance of the varactor diode is utilized as the idlercircuit. Generally, however, the first self-resonant frequency is lowerthan the frequency which minimizes the noise factor. This prevents thefull utilization of the capability of the varactor diode in relation tothe noise characteristic.

FIG. 3a illustrates an embodiment of the microwave circuit of theinvention utilized in a parametric amplifier. In the parametricamplifier of FIG. 3a, the second self-resonance of the varactor diode 1is utilized. The waveguide 2 through which the idler frequency cannotpass is equivalently short-circuited at a point 100 at a distance L0,which is a quarter wavelength of the idler frequency, from the varactordiode 1. In accordance with the invention, the idler circuit adjustingelement 15, which may comprise, for example, a screw, is inserted in thewaveguide 2 in the direction of the electric field of said waveguide ata distance L1 from the varactor diode l. The distance L1 is greater thanone quarter wavelength of the idler frequency. The element may be fromone quarter to one half wavelength from the diode 1. The distance L2 isgreater than L1. The end 9 is a screw which is capable of adjusting thepumping circuit without affecting the idler circuit.

The idler equivalent circuit shown in the equivalent circuit of FIG. 3bis thus a parallel resonant circuit 16 having a tuning frequency whichis continuously variable. The parallel resonant circuit 16 replaces theinductance ll of the equivalent circuits of the prior art of FIGS. lband 2b. This indicates that the structure of the parametric amplifier ofFIG. 3a permits the obtaining of a wideband single-humped amplificationcharacteristic when the signal circuit is single-tuned.

FIG. 4a illustrates another embodiment of the microwave circuit of theinvention as utilized in another embodiment of a parametric amplifier.The choke filter 4 is again provided close to the waveguide 2, as in theconventional parametric amplifier shown in FIG. 2a.

The second self-resonance of the varactor diode 1 is utilized. In theidler equivalent circuit of the equivalent circuit of FIG. 4b, theparallel resonant circuits 13 and 16 are mutually coupled, as shown bythe doubleheaded arrow 14. Experimentation has proven that the structureof the embodiment of FIG. 4a may produce a double-humped amplificationcharacteristic, even when the signal circuit is single-tuned.

FIG. 5 illustrates the amplification frequency characteristics ofparametric amplifiers exhibited when the signal circuits aresingle-tuned. In FIG. 5, the abscissa represents the amplified frequencyf in gigahertz and the ordinate represents the gain in db. A curve 17illustrates the characteristic of the parametric amplifier of FIG. 1a. Acurve 18 illustrates the characteristic of the parametric amplifier ofFIG. 2a. A curve 19 illustrates the characteristic of the parametricamplifier of FIG. 3a. A curve 20 illustrates the characteristic of theparametric amplifier of FIG. 4a.

It is evident that the characteristic curves l9 and 20 of the parametricamplifiers of FIGS. 3a and 4a of the invention are superior to thecharacteristic curves I7 and 18 of the conventional parametricamplifiers of FIGS. la and 2a. In accordance with conventional theory,the frequency-gain characteristic of the parametric amplifier of FIG.4a, if the idler has two parallel resonant circuits l3 and 16 ofdifferent resonant frequencies (FIG. 4b), should become a double-humpedcharacteristic with an extremely deep valley when a resistance otherthan the series resistance of the varactor diode 1 is neglected.Actually, however, as shown in the curve 20 of FIG. 5, there is no deepvalley, and when the resonant frequencies of the parallel resonantcircuits l3 and 16 are made to approach each other, the two resonantfrequencies approach each other due to the mutual coupling of saidparallel resonant circuits and a double-humped characteristic with avalley of about 1 db may be provided.

The foregoing cannot be satisfactorily explained by the conventionalconcept of regarding an operation as only either one of the currentexcitation type, wherein an external circuit viewed from a varactordiode may be regarded as a series resonant circuit, or the voltageexcitation type, wherein an external circuit viewed from a varactordiode may be regarded as a parallel res onant circuit. The satisfactoryexplanation for the fact that the valley of the double-humpedamplification fre quency characteristic of the curve 20 of FIG. 5,indicative of the parametric amplifier of FIG. 4a, is not very deep, isthat the two humps primarily indicate the current excitation typeoperation and the valley indicates the combination of the currentexcitation type operation and some voltage excitation type operation. Ifthe idler circuit of the parametric amplifier of FIG. 40 has a circuitloss, the valley of the double-humped characteristic becomes shallow andbecomes, as a whole, a wideband characteristic, and even theamplification frequency characteristic may be obtained. Therefore, inthe case where some deterioration of the noise characteristic isallowable, it is possible to realize a wideband characteristic byincluding a resistor in addition to the idler circuit adjusting elementor screw 15.

In the case of a cooled parametric amplifier, which is required for anextremely low noise characteristic, even if a wideband and a levelamplification frequency characteristic is provided by the use of asingle-tuned idler circuit and a double-tuned signal circuit, the noisecharacteristic is deteriorated at the two ends of the band due to thequality factor Q of the idler resonant circuit. In this case, it is alsopossible to make the entire amplification frequency characteristic leveland lower the maximum equivalent noise temperature within the band bythe series connection of parametric amplifiers of the embodiment of FIG.4a in which the idler circuit is made double-tuned and adjustment ismade so that the noise characteristic may be improved at the two ends ofthe amplified band.

FIGS. 6a, 6b, 6c and 6d explain the principle of operation of themicrowave circuit of the invention. FIG. 6a is a top sectional view andFIG. 6b is a side sectional view, taken along the lines VlBVIB of FIG.6a, of a rectangular waveguide constructed to investigate to what extentthe microwave energy enters the rectangular waveguide through which theidler frequency cannot propagate. The rectangular waveguide of FIGS. 6aand 6b is also for investigating how the equivalent short-circuitsurface of the waveguide is varied when the frequency approaches thecutoff frequency of said waveguide.

The rectangular waveguide is, as shown in FIG. 6a,

reduced in its horizontal dimensions in the electric field directionfrom the dimension of a normal waveguide 21 by indentations or steps andis reduced in its vertical dimension by a taper, as shown in FIG. 6b.The portion of the wide wall on the left side of a plane 22 vertical tothe axis VlBVIB of the waveguide is reduced symmetrically about saidaxis, as shown in FIG. 6a. A pair of idler circuit adjusting elements orscrews 23 and 24 are inserted in a waveguide 25 of reduced dimensionsextending from the waveguide 21. The idler circuit adjusting elements 23and 24 are spaced from each other and are inserted in the direction ofthe electric field.

The idler circuit adjusting element 23 is spaced from the plane 22 by adistance L3 and the idler circuit adjusting elem ent 24 is spaced fromthe plane 22 by a distance L4. The idler circuit adjusting element 23extends into the waveguide 25 a distance d1 and the idler circuitadjusting element 24 extends into the waveguide 25 a distance d2. Whenmicrowave frequencies at the cutoff region in the waveguide 25 areapplied from the waveguide 21 to the waveguide 25, and the equivalentshort-circuit surface is spaced from the plane 22 a distance L, theforemost end of a magnetic field 26 of the microwave arrives at aposition in the waveguide 25 spaced from the plane 22 by more than theequivalent short-circuit surface.

FIG. 6c illustrates the variation of a distance L5 (FIG. 6a) between theequivalent short-circuit surface and the plane 22 in the situation wherethe length (11 and d2 of the inserted portions of the idler circuitadjusting elements 23 and 24 within the waveguide 25 are both zero andthe microwave frequency fis increased and equal to the cutoff frequencyfc of the waveguide 25. FIG. 60. shows a curve 27 which indicates therelation between the length d1 of the inserted portion of the idlercircuit adjusting element 23 provided at the position of three eighthsfree space wavelength of the microwave frequency fl) and the distance L5between the equivalent short-circuit surface and the plane 22.

In FIG. 6d, a curve 28 shows the reflection coefficient. A curve 29 ofFIG. 6d shows the relation between the length d2 of the inserted portionof the idler circuit adjusting element 24 at the position of threequarters free space wavelength of the microwave frequency f and thedistance L5. As illustrated by the curve 29 of FIG. 6d, series resonanceoccurs when the length of the inserted portion of the idler circuitadjusting element 23 becomes d0 and the variation of the distance L israpid around d0 and also around the point of resonance, as seen from thecurve 28. Under these conditions, the reflection coefficient 1- alsobecomes small and the circuit loss increases. It is therefore necessaryto avoid the insertion of the idler circuit adjusting element 23 untilthe point of resonance is approached. The curve 29 of FIG. 6d isessentially a straight line, except for the portion very close to thepoint of resonance, which indicates that the microwave energy offrequency f0 does not reach the idler circuit adjusting element or screw24.

In FIG. 60, the abscissa represents the frequency in gigahertz and theordinate represents the distance in mm. In FIG. 6d, the abscissarepresents the distance d1 and d2 in mm and the ordinate represents thedistance in mm and the reflection coefficient 7. FIG. 6c

illustrates a curve 31 which is intersected at a point 32 by a distanceL0. In FIG. 6d, the curve 27 is intersected at a point 33 by thedistance L0.

It has been found by actual measurement that the distance L between theequivalent short-circuit surface and the plane 22 (FIGS. 6a and 6b) inthe waveguide 25 (FIGS. 6a and 6b) of the same dimensions as thewaveguide 2 of FIGS. 3a and 4a under the idler frequency with which theparametric amplifiers of FIGS. 3a and 4a operate, is about a quarterwavelength of the free space wavelength. This illustrates that the idlerelectromagnetic field distribution within the waveguide 2 of FIGS. 3aand 4a is similar to the electromagnetic field distribution within thewaveguide 25 of FIGS. 6a and 6b in which the varactor diode (not shownin FIG. 6a or 6b) is provided in the plane 22.

When the distance L1 (FIGS. 3a and 4a) is greater than the distance L4(FIG. 6a) the pump circuit is adjustable by the movement of the variableshort-circuit end 9 (FIGS. 3a and 4a) without affecting the idlercircuit. The idler circuit and the pump circuit may then be adjustedwith great facility by adjusting the idler circuit by the idler circuitadjusting element or screw 15 (FIGS. 3a and 4a) and then adjusting thepump circuit by the variable short-circuit end 9 (FIGS. 3a and 4a). Theidler circuit of a parametric amplifier may therefore be designed bydetermining the range within which the distance L5 between theequivalent short-circuit surface and the plane 22 (FIG. 6a) is varied bythe idler circuit adjusting element or screw 15 (FIGS. 3a and 4a) inview of the curve 27 of FIG. 6d, determining a distance L0 under thesecond self-resonant frequency of the varactor diode at a magnitudeequal to one quarter free space wavelength, selecting Afin FIG. 60 at asuitable magnitude smaller than that of the signal frequency in order todetermine the cutoff frequency fc, and determining the dimensions of thewaveguide 2 (FIGS. 3a and 4a).

In the aforedescribed embodiments of the invention illustrated in FIGS.3a and 4a, the idler circuit adjusting element or screw 15 is providedon the side of the varactor diode 1 facing the variable shortcircuit end9. Exactly the same adjustment effect may be provided by inserting theidler circuit adjusting element or screw 15 on the side of the varactordiode 1 facing the waveguide 3. In such case, the screw 15 is placed adistance L2 from the varactor diode 1. Two idler circuit adjustingelements or screws may be provided, one on each side of the varactordiode 1. The provision of the idler equivalent short-circuit surface atthe position of the waveguide 2 in FIG. 3a spaced from the varactordiode 1 less than a quarter wavelength of the idler frequency isequivalent to the connection of an inductance as the idler externalcircuit of said varactor diode. In such case, the idler frequency may bemade higher than the second self-resonant frequency of the varactordiode 1.

of the parametric amplifier of the invention with an inductanceconnected to the idler circuit as an external circuit of said idlercircuit. In the equivalent circuit, an inductance 34 is constituted by alength L6 which is smaller than a quarter wavelength. A minute reactance35 of the equivalent circuit is constituted by a length L7 approximatinga half wavelength or less. At an eighth wavelength the sum of theinductance 34 and the minute reactance 35 is positive, the externalcircuit of the varactor diode 1 is inductive and the resonant frequencyof the entire circuit is greater than fr2.

FIG. 8 illustrates a curve 36 which shows the relation between the idlerfrequency of the idler circuits of FIGS. 2a and 7 and the bandwidth ofthe idler circuit of the parametric amplifier. In FIG. 8, the abscissarepresents the idler frequency in gigahertz and the ordinate representsthe bandwidth of the idler circuit in megahertz and the noisetemperature in degrees. Kelvin. In FIG. 8, curves 37, 38 and 39illustrate the relation between the idler frequency and the noisetemperature at the ends of specific bands. The relation is avail ablewhen the signal circuit is double-tuned and the gain characteristic islevel within the bands. The amplified bandwidths of the'curves 37, 38and 39 are widened in that order and the idler frequency at whichthenoise temperature becomes a maximum is small.

In FIG. 9, the abscissa represents the single frequency in gigahertz andthe ordinate represents the noise temperature in degrees Kelvin and thegain in db. FIG. 9 illustrates the gain-frequency characteristics andthe noise temperature-frequency characteristics at idlerfrequenciesfil,fi andfi2 (FIG. 8) corresponding to points 41, 42 and 43of the curve 39 of FIG. 8. In FIG. 9, a curve 44 shows the gaincharacteristics, and curves 45, 46 and 47 show the noise temperaturecharacteristics corresponding to the points 41, 42 and 43 in FIG. 8. Thenoise temperatures at the ends fsl and fs2 (FIG. 9) of the band become aminimum when the idler frequency is fi0. By determining the gainbandwidth from these curves, it is possible to select the idlerfrequency at the band ends at which the noise temperatures become aminimum.

FIG. a is a vertical sectional view of a practical embodiment of theparametric amplifier of the invention having an idler circuitto which aninductance is connected as an external circuit of the idler circuit.FIG. 10b is a cross-sectional view taken along the lines XB-XB of FIG.10a. In FIGS. 10a and 10b, the parametric amplifier comprises a varactordiode 48. A pumping electric power supply waveguide 49 has a taperedportion 51 extending into a waveguide 52 through which the pumpfrequency may be passed, but which prevents the passage of the idlerfrequency. A variable short-circuit end 53 is provided for matching thepump electric power.

In the parametric amplifier of FIGS. 10a and 10b, an impedancetransformer 54 (FIG. 10a) is connected in the signal frequency system.The transformer 54 varies the coupling coefficient of the signalcircuit. A radial choke or choke filter 55 (FIG. 10a) chokes the pumpelectric power and a radial choke 56 chokes the idler electric power. Adisc 57 (FIG. 10a) provides an elec' trical length L8 (FIG. 10a) fromthe varactor diode 48 to the radial choke 56. The disc 57 has a concaveportion 58 formed therein. The electrical length L8 is variable inaccordance with the variation of the concave portion 58 of the disc 57even when the thickness of said disc is constant.

A radial choke may be regarded as a quarter wave length line with ashort-circuited end. Therefore, when the length L8 is about a quarterwavelength, the coaxial line between the varactor diode 48 and theradial choke 56, viewed at the idler frequency, is equivalent to thelength L7 of the half wavelength in FIG. 7. The waveguide 52, throughwhich the idler frequency cannot pass, may be regarded as a line of aspecific length with a short-circuited end and said waveguide is viewedfrom the varactor diode 48. The waveguide 52 therefore becomesequivalent to the length L6 of FIG. 7. In this case, the distance L6from the equivalent short-circuit surface to the varactor diode 48 isvaried by the width W of the waveguide 52.

The width W of the waveguide 52 is therefore selected so that the idlercircuit of FIG. 4a may substan tially resonate at the optimum idlerfrequency fi0 provided by FIG. 8 and the fine adjustment may be providedby the variation of the depth of the concave por tion 58 of the disc 57.This may be achieved with facility and rapidity by exchangeablyutilizing a plurality of discs 57 having concave portions 58 ofdifferent depths.

FIG. 11a shows another embodiment of the parametric amplifier of theinvention utilizing a low pass filter 59 for preventing the leakage ofthe pump electric power and the idler electric power to the coaxial lineside of the waveguide 52. Fine adjustment may be provided in the lowpass filter 59 by varying the depth L9 of the concave portion 61 of thelow pass filter element 62 of the low pass filter 59.

FIG. 1 lb shows still another embodiment of the parametric amplifier ofthe invention. In the embodiment of FIG. 11b, a holding stand or support63 supports or holds the varactor diode 48. The support 63 has a concaveportion 64 formed therein. Fine adjustment is provided by varying thedepth L10 of the concave portion 64 of the support 63. The low passfilter 59 has a standard low pass filter element 65.

FIG. 12 illustrates the relation between the width W of the waveguideand the external reactance. In FIG. 12, the abscissa represents thewidth W of the waveguide in mm and the ordinate represents the externalreactance XT in ohms. The external reactance equals QL34 XC35 (FIG. 7).Curves 66, 67 and 68 of FIG. 12 show the relation between the entireexternal reactance XT of the idler circuit and the width W of thewaveguide in a parametric amplifier in which the length L8 of FIG. orthe depth L9 of FIG. 11a and L10 of FIG. 11b have a specific constantvalue. The inductance L34 (FIG. 7) required for the optimum idlerfrequency is determined by the width W of the waveguide determined fromFIG. 12. Fine adjustment may be pro vided by the variation of thereactance XC35 (FIG. 7). It should be noted that the width W of thewaveguide is limited to a range WP to Wi, because when said width isless than WP, the pump frequency cannot pass through the waveguide, andwhen said width exceeds Wi, the idler frequency is passed through thewaveguide.

As hereinbefore described, the microwave circuit of the inventionutilizes a pump supply waveguide through which the idler frequencycannot be passed and which has an equivalent short-circuit surface,located at a position spaced from. the varactor diode by a quarterwavelength of the idler frequency. An idler circuit adjusting elementsuch as, for example, a screw, is provided at a position spaced from thevaractor diode by a quarter wavelength of the idler frequency. Theforegoing features result in a simple structure of the microwavecircuit, facility and rapidity of adjustment and electricalcharacteristics which are superior to those of the conventionalcircuits.

Furthermore, in accordance with our invention, the parametric amplifiermay be operated at a point at which the noise characteristic of thevaractor diode is the most suitable or excellent by utilizing an idlerfrequency higher than the self-resonant frequency of the varactor diode.The energy of the idler frequency may be confined to an area very closeto the varactor diode and the amplification bandwidth may be widened.The idler circuit may be adjusted with facility and rapidity. A specialfeature of the invention is that a considerable suitable effect may beprovided by the application of the invention to a parametric amplifieroperated at room temperature in order to provide a sufficiently widebandwidth and to reduce the noise temperature at the ends of the band toa minimum.

The aforedescribed idler circuit of the invention is also applicable toa varactor diode multiplier having an idler resonant circuit and a chokefilter of the image frequency of a frequency converter comprisingsemiconductor elements, and the like.

While the invention has been described by means of specific examples andin specific embodiments, we do not wish to be limited thereto, forobvious modifications will occur to those skilled in the art withoutdeparting from the spirit and scope of the invention.

We claim:

1. A microwave circuit, comprising a waveguide;

a coaxial line having an axis perpendicular to the waveguide;

a diode mounted in the waveguide at the intersection of the coaxial lineand the waveguide, thereby providing the waveguide with a region whichprevents the passage of the idler frequency, the waveguide havingdimensions determined to provide an equiv alent short-circuit surface inthe waveguide at a distance of about a quarter wavelength from thediode, the waveguide exhibiting an inductance preventing the passage ofthe idler frequency;

an idler circuit including the inductance of the waveguide; and

a screw for adjusting the idler frequency provided in the waveguide at aposition spaced from the diode by more than a quarter wavelength of theidler frequency.

2. A microwave circuit as claimed in claim 1, wherein the idler circuitcomprises the inductance of the waveguide preventing the passage of theidler frequency and a reactance exhibited by the coaxial line.

3. A microwave circuit as claimed in claim 1, wherein the resonantfrequency of the idler circuit is selected to be higher than theself-resonant frequency of the diode.

4. A microwave circuit, comprising a waveguide which prevents thepassage of the idler frequency, the waveguide exhibiting an inductance;

a coaxial line extending from the waveguide substantially perpendicularto the waveguide;

a diode mounted in the waveguide at the intersection of the waveguideand the coaxial line;

short-circuit means providing an equivalent shortcircuit surface in thewaveguide at approximately a quarter wavelength from the diode;

an idler circuit comprising the inductance exhibited by the waveguide;idler circuit adjusting means in the waveguide at more than a quarterwavelength of the idler frequency for adjusting the idler frequency;support means supporting the diode in the waveguide and a low passfilter adjacent the diode between the waveguide and the coaxial line;and

fine adjusting means for the idler circuit comprising a concave grooveformed in the support means in the waveguide adjacent the diode, thefine adjusting being determined by the depth of the groove.

5. A microwave circuit as claimed in claim 4, wherein the diode is avaractor diode.

1. A microwave circuit, comprising a waveguide; a coaxial line having anaxis perpendicular to the waveguide; a diode mounted in the waveguide atthe intersection of the coaxial line and the waveguide, therebyproviding the waveguide with a region which prevents the passage of theidler frequency, the waveguide having dimensions determined to providean equivalent short-circuit surface in the waveguide at a distance ofabout a quarter wavelength from the diode, the waveguide exhibiting aninductance preventing the passage of the idler frequency; an idlercircuit including the inductance of the waveguide; and a screw foradjusting the idler frequency provided in the waveguide at a positionspaced from the diode by more than a quarter wavelength of the idlerfrequency.
 2. A microwave circuit as claimed in claim 1, wherein theidler circuit comprises the inductance of the waveguide preventing thepassage of the idler frequency and a reactance exhibited by the coaxialline.
 3. A microwave circuit as claimed in claim 1, wherein the resonantfrequency of the idler circuit is selected to be higher than theself-resonant frequency of the diode.
 4. A microwave circuit, comprisinga waveguide which prevents the passage of the idler frequency, thewaveguide exhibiting an inductance; a coaxial line extending from thewaveguide substantially perpendicular to the waveguide; a diode mountedin the waveguide at tHe intersection of the waveguide and the coaxialline; short-circuit means providing an equivalent short-circuit surfacein the waveguide at approximately a quarter wavelength from the diode;an idler circuit comprising the inductance exhibited by the waveguide;idler circuit adjusting means in the waveguide at more than a quarterwavelength of the idler frequency for adjusting the idler frequency;support means supporting the diode in the waveguide and a low passfilter adjacent the diode between the waveguide and the coaxial line;and fine adjusting means for the idler circuit comprising a concavegroove formed in the support means in the waveguide adjacent the diode,the fine adjusting being determined by the depth of the groove.
 5. Amicrowave circuit as claimed in claim 4, wherein the diode is a varactordiode.